1. Field of the Invention
The present invention relates in general to a line sensor and a radiation-image readout apparatus, and in particular to a line sensor having a broad-width light-receiving portion for readout and a radiation-image readout apparatus in which said line sensor is to be deployed.
2. Description of the Related Art
There are known line sensors wherein the light-receiving portion comprises a photodiode divided into a plurality of element regions at a predetermined pitch along the lengthwise direction thereof. These line sensors, as shown in the schematic drawing of the line sensor depicted in FIG. 8, are provided with a photodiode 1, which is divided along the lengthwise direction thereof (the direction X in FIG. 8) into a plurality of element regions 4, a transfer gate 2, and a charge transfer device 3 such as a CCD or the like for transferring through the transfer gate 2 to an external portion the charge generated at each of the element regions 4 of the photodiode 1. According to the operation of a line sensor of the configuration described above: a quantity of electrical charge is generated at each element region 4 of the photodiode 1 corresponding to the quantity of light energy received thereon; these charges are transferred via the transfer gate 2 to the charge transferring device 3, and are again transferred by the transferring device 3 through an amplifier or other signal processing device (not shown) to an external portion; such line sensors are widely used in a number of fields in apparatuses for detecting light. For example, there are radiation-image readout apparatuses for reading out the radiation-image data that has been cumulatively recorded on a stimulable phosphor sheet and which comprise a built-in line sensor for detecting radiation.
The aforementioned stimulable phosphor sheet utilizes a layer containing stimulable phosphors (storable phosphors), which upon the irradiation thereof with a radiation store a portion of the radiation energy thereof, and emit upon the subsequent irradiation thereof with a visible light, a laser light or other excitation light, a stimulated emission corresponding to the radiation energy stored therein; said layer being formed on a substrate. These stimulable phosphor sheets are widely used in conjunction with radiation-image recording/reproducing systems, wherein: the radiation-image data of a human body or other subject of photographing through which radiation has been passed is temporarily stored on a stimulable phosphor sheet; said stimulated phosphor sheet is scanned with a laser or other excitation light causing a stimulated emission to be emitted therefrom; said stimulated emission is photoelectrically read out by a photoelectric readout means to obtain an image signal; and on the other hand, said stimulable phosphor sheet is then irradiated with an erasing-light to erase the radiation energy remaining thereon (refer to Japanese Unexamined Patent Publication Nos. 55(1980)-12429, 55(1980)-116340, 56(1981)-104645, etc.).
Here, according to a radiation-image readout apparatus to be employed in the above-described radiation-image recording/reproducing system: a line light source is used as the excitation light source for irradiating the sheet with an excitation light in a line form; a line sensor provided with a plurality of photodiodes arranged in side by side along the lengthwise direction (hereinafter referred to as the main scanning direction) of the line-form portions of the sheet which have been irradiated with the excitation light by the line light source is employed as a photoelectric readout means; and a scanning means is provided for moving the line light source and the line sensor relatively from one end to the other of the sheet in the direction substantially perpendicular to the main scanning direction (hereinafter referred to as the sub-scanning direction) of the sheet; wherein the aim of this configuration is to obtain a shortening of the readout time and a reduction in the size and cost of the apparatus, have been proposed (refer to Japanese Unexamined Patent Publication Nos. 60 (1985)-111568, 60(1985)-236354, 1(1988)-101540, etc.).
Because the stimulated emission emitted from the stimulable phosphor sheet is extremely faint, it is desirable that as much of the stimulated emission as possible is received when readout thereof is performed, and there have been calls for enlarging the size of the area of the light-receiving portion of the line sensor serving as a light detecting apparatus; that is to say, there have been calls for an enlargement of the width of the photodiode portion (the size in the width direction perpendicular to the lengthwise direction of the photodiode portion).
Further, among stimulable phosphor sheets there are those that are of a comparatively large area, and when readout of a stimulable phosphor sheet having a large area of this type is performed, there are cases in which the read out is performed at a considerably large pixel size in order to shorten the readout time, etc. and there are a calls for a line sensor having a broad-width light-receiving portion (photodiode) for adequately reading out the stimulated emission emitted from the large-sized pixel regions such as those described above.
These calls have not been limited to the read out of stimulable phosphor sheets; similar calls have also been made with respect to the read out of indirect X-ray panels, low-brightness emission images, the surface detection of sheets, etc.
However, if the width of the photodiode portion is widened, the time required for transferring the entire charge cumulatively stored thereon by a charge transferring device becomes longer, that is, the readout time of the charge signal is lengthened.
Therefore, the charge stored on the photodiode portion of the line sensor upon the emission of a stimulated emission from a stimulated phosphor sheet, for example, cannot be readout within a predetermined time, and if the read out is attempted to be completed in a shortened time, a problem arises in that the charge cannot be sufficiently read out and the quality of the read out signal is not good.